Sarah Palin set off a storm of speculation about an imminent White House bid when she said yesterday that she was stepping down as governor of Alaska.

Americans were stunned by the surprise announcement, made from her home in Wasilla, Alaska, on the eve of Independence Day celebrations.

There has been intense speculation in recent weeks that Mrs Palin was considering running for the Republican nomination in 2012, bolstered by heavy hints she dropped earlier this week in the guise of an interview about jogging.

Few expected her not to see out her first term as governor where, despite her polarising effect, she was seen as a shoo-in for re-election.

Mrs Palin’s announcement that she will stand down on July 25, handing the reins to the state’s Lieutenant Governor, had some commentators questioning whether another scandal surrounding herself and her family was about to break, after the 2008 campaign revelations about the pregnancy of her teenage daughter and an embarrassing ethics investigation into allegations she sacked a state official over a family feud.

The first investigation by the state legislature into the scandal — popularly known as Troopergate — found her guilty of breaching ethics, prompting Mrs Palin to order a second investigation by a special counsel which cleared her of wrongdoing.

The resignation also sparked a flurry of speculation that she might seek a Senate seat in 2010 as a prelude to a White House run in 2012.

Critics branded it a high-risk strategy for a future in public life, inviting criticism that she is not capable of finishing the job she started. Mrs Palin has a reputation for doing things her way and refusing to take advice of more experienced political operatives.

Much of the criticism that dogged her during her vice-presidential campaign in November centred on her parochialism and lack of national and international experience — something she might seek to improve on a national stage.

In a pointed reference to her recently expanded international experience, she said that her decision had been bolstered by a trip to visit American troops serving in Kosovo, and to the US military hospital in Landstuhl, Germany, where wounded servicemen and women from Iraq and Afghanistan are treated.

Her announcement came days after the publication of a damning Vanity Fair profile in which McCain campaign workers turned on her, blaming her “narcissistic personality disorder” for sinking the campaign.

Todd Purdum, the author, described Mrs Palin’s public life as “an unholy amalgam between Desperate Housewives and Northern Exposure”, a cult Nineties comedy about Alaska and mocked her for once saying: “Believe me, Alaska is a microcosm of America.” “Believe me, it is not,” he wrote.

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